1. Field of the Invention
This invention relates to frequency doubling of light and more particularly to the use of a branching waveguide to generate first order mode frequency doubled light.
2. Description of the Prior Art
Semiconductor diode lasers are used in optical data storage systems. The gallium-aluminum-arsenide (GaAlAs) diode laser is one example and it generates light in the near-infrared range (860 nanometers wavelength). The light from the laser is focused onto a spot on the optical disk in order to record each bit of data. The spot size is equal to .lambda./2(N.A.), where .lambda. is the wavelength of the light and (N.A.) is the numerical aperture of the focusing lens. In typical systems, the (N.A.) is approximately 0.5 and the resulting spot size is 860 nanometers in diameter.
It is apparent that if the wavelength of the laser light can be cut in half, the diameter of the spot size will also be cut in half and the overall density of the optical disk will be quadrupled. Unfortunately, laser diodes which produce light in the blue range (430 nanometers in wavelength) are not yet available. Research in this area has concentrated on ways to convert the infrared light from the laser diode into blue light.
One technique to convert light to a higher frequency is known as second harmonic generation (SHG). Light is passed through a nonlinear crystal, such as potassium niobate (KNbO.sub.3) and the second harmonic light (light at twice the frequency of the fundamental light) is generated.
These methods of SHG typically use bulk crystals. The power of the blue light generated is proportional to 1.sup.2 /A, where 1 equals the length of the crystal and A equals the area of the focussed beam. Optimally, the length should be as long as possible and the area of focus very small. The problem is that with traditional optical focussing in bulk crystals, the beam can be focussed in a small area for only a short length before the light begins to diverge. Conversely, the light can be focussed for a longer length of the crystal, but at a much larger area of focus. The result is that the SHG process is very inefficient. For example, a hundred milliwatts of input fundamental frequency light results in only 10 microwatts of output second harmonic light. This is not enough power for use in optical storage systems.
One solution to this problem is to do the SHG process in a nonlinear crystal waveguide. Here the light can be confined to a small area (the cross section of the waveguide) for the entire length of the waveguide.
One problem with SHG in waveguides is that the second harmonic light is generally produced in a plurality of higher-order modes of the waveguide. This occurs because dispersion of the waveguide causes the effective index of the lowest-order mode at the second harmonic to be higher than that of the lowest order mode of the fundamental light. Phase matching can usually be obtained in simple planar or channel waveguides between the lowest order mode at the fundamental frequency light and the higher order modes of the second harmonic light. Typically, the waveguide will be designed to be a single mode at the fundamental wavelength and will support a few (approximately 2 to 4) modes at the second harmonic wavelength.
One method for conversion of the higher-order second harmonic mode to a lower-order mode is the use of a directional coupler which is designed to couple a higher-order mode in one waveguide to the lowest-order mode in a second waveguide which is parallel to the first waveguide. This coupling is achieved by designing the waveguides such that the effective index for the higher-order mode in the first guide is identical to the effective index for the lowest-order mode in the second guide. The evanescent field of the higher-order mode in the first guide will then preferentially excite the lowest-order mode in the second guide. Fabrication of such a coupler requires precise knowledge of how the physical parameters such as width and index profile of the waveguide will affect the effective index of modes within the waveguide, and in addition, requires the ability to precisely control fabrication conditions to achieve the desired physical parameters. The low fabrication tolerances required make the directional coupler impractical. It may be possible to overcome this difficulty by adjusting the effective indexes electro-optically, but this would require a far more complex structure and would demand precise control of the waveguide.
What is needed is an inexpensive nonlinear crystal waveguide which produces second harmonic generated light in a first order mode.